speculative fix for destroyed activity case

It's possible for the two-pane conversation->list animation to
finish after the activity is already destroyed (apparently).
Choreographer is not aborting early, so we must check for a dead
Activity and avoid work (which may depend on Activity) in
response to onAnimationEnd.

Bug: 6931878
Change-Id: Ifeb4ac6664e0db18b263ae201a20d0811971f347
3 files changed